1. Field of the Invention
The present invention generally relates to integrated circuit devices and, more particularly, to a single, multifunctional, integrated circuit input scheme which is user-programmable.
2. Description of the Related Art
Integrated circuit (IC) miniaturization has progressed to the stage where, in very large scale integration (VLSI) technologies, there are in excess of a million devices on an integrated chip which is typically 0.3 to 0.5 square inch in surface area.
While functional density and capability at the IC level has increased dramatically, one limitation on the flexibility of IC use has remained the number of input and output pins available on the chip carrier package. The circuit designer is limited as to the functions which the device can perform by the number of input control signals and output signals that will be available based on the design of the carrier package. Examples of typical dual-inline package input/output requirements for several programmable read only memory (PROM) ICs are shown in FIG. 1.
One solution is to use packages with more pins. Pin grid array packages, such as shown in FIG. 2, provide a higher input/output capability. However, this solution is at the expense of miniaturization and problems such as chip-to-chip interconnect distances and pin-to-socket alignment difficulties. While suitable for chips such as programmable logic arrays (PLA) used, for example, as simple state-machines, such packaging would be unacceptable for higher logic devices such as memory chips used in compact, powerful, high-speed computers.
Another solution has been to provide the user with the option to implement a variety of input functionalities through fabrication process modification. Typically, separate metallization masks are used to alter the interconnect pattern of the completed integrated circuit. This, of course, is a customization during fabrication which cannot be altered thereafter.
A simple "programmability" solution has been to provide the user with an "either/or" choice for a particular pin. For example, the devices of FIG. 1, manufactured by the current assignee, Advanced Micro Device, Inc., Sunnyvale, Calif., can have a programmable output enable pin and/or a programmable register initialize pin. The unprogrammed state of these pins is asynchronous operation. Should the system in which they are used require, either function may be changed to a synchronous mode of operation by programming an architecture word. For example, the initialize function uses a programmable word which is loaded into the output data registers to change the device to synchronous operation. This is a very limited, narrow application solution to the problem at hand.
Hence, there is a need for increasing the flexibility of the available pins of an IC package.